1. Field of Invention
This invention relates to highly active catalysts useful for syngas generation, and more particularly to promoted calcium-aluminate supported catalysts, wherein the promoter is selected from the group consisting of titanium, zirconium, yttrium, niobium, elements of the lanthanum-series, such as, without limitation, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, ytterbium and combinations thereof. The catalyst comprises an active metal selected from the group consisting of nickel, cobalt, rhodium, ruthenium, platinum, palladium, iridium and combinations thereof. The catalyst is highly active, stable and resistant to coking when used in producing synthesis gas, especially low H2/CO synthesis gas. A process of manufacture of the catalysts and a process of use of the catalysts are also disclosed.
2. Background Art
Production of synthesis gas or syngas (various blends of gases generally comprising hydrogen and carbon monoxide) is an important process step in the manufacture of numerous chemicals, such as ammonia and methanol. It is also useful in numerous other commercial and developmental processes, such as iron ore reduction, Fischer-Tropsch synthesis and other gas-to-liquid processes. Many of the syngas plants produce the syngas by steam reforming of light hydrocarbons, usually natural gas, and the syngas commonly has an H2/CO ratio larger than 3. Typically, these plants employ a supported nickel catalyst, usually nickel on an alpha-alumina support or nickel on a promoted-alumina support.
However, a problem that often occurs with reforming reactions is an enhanced likelihood of coking or carbon formation on the catalysts. Several solutions have been proposed to address the coking problem. For example, a large excess of H2O in the reformer feed stream can be applied in applications where H2 is the target product and CO is only a lower value by-product, such as in ammonium synthesis or hydrogen production. The excess of H2O generates more H2 via the water-gas-shift reaction. However this solution is not suitable for applications where a low H2/CO ratio syngas is required, such as for the gas-to-liquid processes.
Another process for limiting carbon formation on nickel catalysts during reforming reactions utilizes sulfur in the feed stream. In this process—referred to as passivation—sulfur poisons some, but not all, of the nickel sites on the catalyst and produces a reforming catalyst which retains sufficient active sites to be useful for syngas production at lower H2/CO ratios. The amount of sulfur that is present in the feed stream must be carefully controlled so that the catalyst retains sufficient activity for the reforming reaction, and the process often requires a substantial quantity of catalyst in the bed. Further, if sulfur is a poison for downstream catalysts, as in Fischer-Tropsch synthesis processes, the sulfur must be removed before it can travel downstream.
The coking risk may be reduced by modifying the catalyst formulation. For example, U.S. Pat. No. 5,753,143 proposes the use of a noble metal catalyst. It is well known that noble metal catalysts have higher coke formation resistance compared to conventional steam reforming catalysts that merely utilize nickel, but these noble metal catalysts are quite expensive, especially with the large quantity of catalysts that is conventionally utilized for this type of reaction. Morioka has addressed the coking problem by the use of high dispersion of metal species over the surface of the catalyst, such as various types of double hydroxide catalysts. U.S. Pat. No. 4,530,918 teaches a nickel on alumina catalyst with a lanthanum additive.
Conventional steam reforming nickel on alpha-alumina catalysts may include additives to enhance their performance and to reduce the coking problem. For example, alkali compounds may be added to steam reforming catalysts to reduce carbon formation but because of their potential migration during high temperature processing the alkali metals can adversely impact downstream operations. Magnesia has also been added to steam reforming catalysts to suppress carbon formation, but magnesia promoted catalysts are hard to reduce and maintain in a reduced state.
Calcium oxide as a promoter to the nickel on alumina steam reforming catalyst has been successfully used commercially. Better coking resistance and overall performance compared to the alpha-alumina catalyst has been reported. But calcium-rich calcium aluminates in a steam reforming catalyst are not desirable because they can hydrate readily and damage the integrity of the catalyst pellets. Further, as is known in the art, calcium aluminate based catalysts need to be treated to eliminate calcium-rich calcium aluminate phases, such as 12CaO.7Al2O3and 3CaO.Al2O3, and the aluminum-rich phases, such as CaO.Al2O3, CaO.2Al2O3 and CaO.6Al2O3, need to be stabilized before nickel impregnation.
A higher calcination temperature can force the calcium aluminates to be transformed to the aluminum-rich phases but also cause surface sintering that is not desirable for most catalytic applications. A promoter that facilitates or stops the phase transformation process of calcium aluminates will make the catalyst more stable thermally and catalytically. A phase transfer facilitator would result in the more stable and aluminum-richer calcium aluminate phases with minimized surface sintering while a phase transfer stopper would stabilize the calcium aluminates at aluminum-leaner phases.
Thus, there is still a need for more active, more coking-resistant, and more stable catalysts for syngas generation, especially for the processes that directly produce syngas with H2/CO ratio less than 2.3.